II. Pathophysiology
- Total Body Sodium Deficit exceeds water losses
- Decreased Extracellular fluid volume- Increased proximal tubule fluid reabsorption
- Decreased distal segment flow where dilution occurs
 
- 
                          Hypovolemia stimulates non-osmotic fluid conservation- Thirst
- ADH secretion
 
III. Causes: Non-Renal Losses (Sodium appropriately conserved, Urine Sodium <20 meq/L)
- Gastrointestinal losses- Diarrhea
- Vomiting- Also causes Metabolic Alkalosis
- Triggers renal bicarbonate excretion, and concomitant renal Sodium losses
 
 
- Third space losses
- Skin Losses- Severe burns
 
IV. Causes: Renal Losses (Renal inappropriate Sodium losses, Urine Sodium >20 meq/L)
- 
                          Diuretics (Thiazide Diuretics, Loop Diuretics)- Sodium loss with overdiuresis triggers ADH release
- Increased ADH results in free water retention
 
- 
                          Renal Tubular Acidosis
                          - Hyperchloremic Metabolic Acidosis
- Increased urinary pH
- Fractional Excretion of Bicarbonate >15-20%
 
- Salt-losing Glomerulonephritis- Chronic Renal Insufficiency on Low Sodium Diet
- Severe interstitial Kidney disease- Polycystic Kidney Disease
- Medullary cystic disease
- Chronic Pyelonephritis
 
 
- Mineralocorticoid and Glucocorticoid deficiency
- Osmotic Diuresis (Bicarbonate, Glucose, Ketones)- Excess osmotically active solutes in urine
- Draws increased Sodium and water renal losses
 
- Salt Wasting Nephropathy- Causative agents (esp. Chemotherapy) inhibit epithelial Sodium channels resulting in Polyuria
 
- Cerebral salt wasting (head injuries, Intracranial Hemorrhage)- Rare diagnosis of exclusion
- Associated with loss of ADH excretion and excessive Urine Output and key Urine Sodium loss
- Contrast with SIADH (characterized by water retention)
 
V. Labs
- 
                          Urine Sodium < 20 meq/L- Non-Renal Sodium Loss (e.g. Vomiting, Diarrhea, severe burns)
- Other lab findings- Urine Osmolality >400 mOsm/kg
- Fractional Excretion of Urea <35%- Use instead of Urine Sodium in patients on Diuretics
- Carvounis (2002) Kidney Int 62(6): 2223-9 [PubMed]
 
 
 
- 
                          Urine Sodium > 20 meq/L- Renal Sodium Loss (e.g. Diuretics, RTA, Adrenal Insufficiency)
- Other lab findings- Urine Osmolality <400 mOsm/kg
 
 
VI. Differential Diagnosis
- Often difficult to distinguish Iso- from Hypovolemic
- See Isovolemic Hypoosmolar Hyponatremia
VII. Management
- See Hyponatremia Management
- Stop all Diuretics
- Correct non-renal fluid losses
- Replace Sodium deficit- Calculate Total Body Sodium Deficit
- Use Normal Saline (0.9% = 150 meq/L)
- Replace one third Sodium deficit over first 6-8 hours
- Replace remaining Sodium deficit in next 24-48 hours
 
VIII. References
- Edwards, Yang and Mehta (2025) Crit Dec Emerg Med 39(9): 25-33
- Kone in Tisher (1993) Nephrology, p. 87-100
- Levinsky in Wilson (1991) Harrison's IM, p. 281-84
- Rose (1989) Acid-Base and Electrolytes, p. 601-38
- Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
- Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]
